GE C70 Instruction Manual page 453

CHAPTER 8: APPLICATION OF SETTINGS
The maximum rated voltage across the healthy bottom section:
The voltage distribution factor for the healthy perfectly balanced bank for the 87V-2 and 87V-3 elements:
The impedance of the string for the failed capacitors elements (for one failed element, n
sf
Z = Z
C
The voltage at the tap in the section with one failed capacitor element in the string:
The voltage of the top section with one capacitor element failed:
The overvoltage at the affected capacitors with one capacitor element failed:
OV
The differential voltage between two taps with one failed capacitor element in the upper string, assuming K = 1 for 87V-1,
in per-unit values is:
V
diff tap (
The differential voltage between the bus voltage and the tap with one failed capacitor element in the upper string,
assuming K = 1, for 87V-3 in per-unit values is:
V =
diff string ( )
The unbalance current in the window type CT between two strings with one failed capacitor element in the upper string:
V
I = ----------------------- -
unbal
sf
Z
C
The following table summarizes the calculated values for the number of failed capacitors used to set the protection
elements.
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
b
×
V Z
81515.37 V 4.08 Ω
max C
V = ---------------------- = ----------------------------------------------- -
bottom
1894.92Ω
t
Z
C
V
81340.40 V
top
K = ---------------- - = ------------------------- -
tap
V 175.37 V
bottom
e b
( × ) Z ×
N n n + = 45.01Ω
–
C f C
b
V × Z
81515.37 V 4.08 Ω
max C
V = ---------------------- = ----------------------------------------------- -
tap2
1849.91 Ω
sf
Z
C
V = V
top max
V × N n ×
81335.74 V 7
f top ( )
= ----------------------------------------- - = --------------------------------------------------------
V × ( N n × n ) 81340.40 V
–
top f
V 1 × V
– 175.374 V 1 179.64 V
tap1 tap2
= =
-------------------------------------- - --------------------------------------------------------- -
)
VT
nom tap ( )
×
V K V
– 175.374 V 463.8194 179.64 V
f top ( ) tap tap2
=
----------------------------------------------- - ---------------------------------------------------------------------------- -
VT
( )
nom bus
V
81515.37 V
max
max
-----------------------
= ---------------------------------- -
–
s 1845.33Ω 50
× Z × CT
CT
C tap
tap
×
= 175.37 V
= 463.8194
f
( × ) 4.08 Ω
7 6 1 + = 1849.41Ω
–
×
= 179.64 V
V
–
tap2
× ×
6
= 1.024 pu
× ( 7 × 6 1 )
–
×
–
= 0.0249 pu
171.59 V
×
–
=
79674.34 V
81515.37 V
---------------------------------- -
= 0.0209 pu
–
× 1890.34Ω 50 ×
SETTINGS EXAMPLE
Eq. 8-13
Eq. 8-14
= 1:
Eq. 8-15
Eq. 8-16
Eq. 8-17
Eq. 8-18
Eq. 8-19
0.0249 pu Eq. 8-20
Eq. 8-21
8
8-15